The present invention relates generally to waveguides and, more particularly, to waveguide couplers having low transmission losses.
Waveguide couplers are used to transfer energy from one waveguide to another. A coupler employing input and output circular waveguides propagating the TE.sub.01 mode is disclosed in U.S. Pat. No. 2,960,670 issued to E. A. J. Marcatili on 15 Nov. 1960, wherein coupling is accomplished by exciting both the TE.sub.01 and TE.sub.02 modes in a gap region between the guides. However, the transmission loss of this coupler is higher than desired for some applications.
One very important operation in microwave transmission systems is the transfer of energy from an evacuated microwave tube to a gas-filled waveguide. Heretofore, a dielectric diaphragm, or window, has been provided at the input port of the waveguide to accomplish this purpose. However, the diaphragm often is unable to withstand the power loading, and breakdown of the diaphragm can occur. One solution to this problem is to flare the waveguide and insert the diaphragm well inside the guide at a larger cross section where the power loading per unit area is less. Unfortunately, the flaring of the guide introduces unwanted higher wave modes. If the waveguide section beyond the diaphragm is tapered to reduce the number of modes, a resonant cavity for the unwanted modes can be formed, resulting in increased losses for the system.
Another important operation in microwave transmission systems is the separation of the output microwave energy from the electron beam in a relativistic electron cyclotron maser or gyrotron. Heretofore, the electron beam has been collected on the walls on the output waveguide. For high power operation, the surface area of the walls must be increased. If the guide is flared and then tapered to increase the surface area, resonant losses from unwanted high wave modes again occur.